The relation between formation temperature and morphology of strain-induced and thermally transformed martensites in various Fe-Ni-C alloys has been studied by means of optical and transmission electron microscopy. The morphology of the strain-induced martensite varied with the formation temperature (i. e., deformation temperature below the Md) even in the The Ms temperature was remarkably depressed in these alloys with decrease in austenitizing temperature. Therefore, the relation between formation temperature and morphology of thermally transformed martensite was clearly determined in the same alloy by utilizing this phenomenon (thermal stabilization of austenite). The morphology of thermally transformed martensite also varied with the formation temperature even in the same alloy. Three types of the martensite were also observed in the same morphologies, and they were similarly formed in the same temperature ranges as the strain-induced martensites, respectively. The main factor determining the morphology of martensite (both the strain-induced and thermally transformed martensites) in Fe-Ni-C alloys was considered to be the formation temperature.

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The authors have previously found that the morphology of martensite in Fe-Ni-0.25%C alloys the transition temperature from the lenticular to the thin plate and the carbon content of Fe-Ni-C alloys was examined by adjusting the carbon content up to 0.9% and the nickel content from 25 to 35%. Furthermore, some features in the progress of transformation to the thin plate martensite from austenite were observed as compared with those of lenticular martensite. The main results obtained are as follows.(1) The transition temperature at which the morphology of martensite in Fe-Ni-C alloys changes from the lenticular to the thin plate rises with increase in carbon content. For example, in (2) The following features in the progress of transformation to the thin plate martensite were observed. (a) During the subsequent cooling, the martensite plate itself thickens in addition to the formation of a new martensite plate in the other part of austenite. (b) The burst phenomenon is not observed. (c) Accommodation slip caused by martensitic transformation is hardly observed in the austenite adjacent to the martensite plate. These phenomena are entirely different from the usual lenticular.

observed. These martensites are usually very thin and have a smooth straight boundary with austenite. Transformation twins run completely through the martensite plate and no mid-rib is observed. The habit plane of this martensite is nearThis plate-like martensite shows a peculiar nature in morphology which can hardly be observed in the usual lenticular martensite. The thin plate-like martensite can cross each other and make kinking or branching. The martensite plate is considered to be thickened by a combination of many parallel thin plate-like martensites with same variant. Consequently, the thick martensite contains many parallel faint boundaries which are boundaries of the original thin plate-like martensite. Furthermore, it seems that this martensite has the nature capable of gradually growing sidewise.

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